// Your runtime beats 87.12 % of java
class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> results = new ArrayList<List<Integer>>();
        if (k > n) return results;

        int[] current = new int[k];
        for (int i=0; i < k; i++) current[i] = i + 1;

        for(;;) {
            List<Integer> result = new ArrayList<>();
            for (int x: current) {
                result.add(x);
            }
            results.add(result);

            int p = k - 1;
            while (p >= 0 && current[p] + k - p > n) {
                p--;
            }

            if (p < 0) break;
            current[p] += 1;
            for (int i = p + 1; i < k; i++) current[i] = current[i - 1] + 1;
        }

        return results;
    }
}

// Runtime: 10 ms, faster than 75.67% of Java online submissions for Combinations.
// Memory Usage: 39.3 MB, less than 82.61% of Java online submissions for Combinations.
class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> results = new ArrayList<List<Integer>>();
        int[] current = new int[k];
        combine(n, k, 0, current, results);
        return results;
    }

    private void combine(int n, int k, int pos, int[] current, List<List<Integer>> results) {
        if (pos >= k) {
            List<Integer> result = new ArrayList<Integer>();
            for (int x: current) {
                result.add(x);
            }
            results.add(result);
            return;
        }

        int from = (pos == 0) ? 1: current[pos-1] + 1;
        for (int i = from; i <= n; i++) {
            current[pos] = i;
            combine(n, k, pos + 1, current, results);
        }
    }
}